Chitin is a linear polysaccharide composed of β-(1-4)-linked 2-acetoamido-2-deoxy-D-glucose units that occur naturally in the exoskeleton of invertebrates, in particular, the carapace of marine crustaceans. Chemical deacetylation of chitin yields chitosan, which is a copolymer of 2-amino-2-deoxy-D-glucose and 2-acetoamido-2-deoxy-D-glucose units.
Soluble chitosan is useful as a coagulant and flocculant in removing impurities such as suspended sediment, proteins, fats, tannins, and metals from aqueous mediums. Soluble chitosan is more commonly used to remove contaminants such as suspended sediment and particulate matter from stormwater running off of construction sites.
Chitosan is insoluble in water, alkali solutions, and most organic solvents. Its effectiveness as a flocculant and coagulant requires it to be dissolved and delivered in soluble form to the aqueous medium containing the material to be coagulated or flocculated. The concentration of soluble chitosan delivered to the aqueous medium containing the material to be coagulated or flocculated affects its effectiveness as a flocculant or coagulant.
Chitosan is soluble in certain solutions of acids at defined acid and chitosan concentrations and slightly soluble to insoluble in other acid solutions. The type of the acid can influence the consistency of the chitosan rendering it paste-like, gelatinous, or more like a pseudo-plastic. In soluble form, chitosan is most commonly found, in commercial applications, to be dissolved in dilute acetic or lactic acid. Because of the high viscosity that chitosan exhibits in dissolved form, commercial chitosan solutions typical exhibit concentrations ranging from 1% (wt./wt.) to 3% (wt./wt.). In flocculation/coagulation applications, the dissolved chitosan in the 1%-3% solutions is metered into the aqueous medium containing the impurity, such as stormwater or industrial water using a metering pump. Higher concentrations (>3%) of dissolved chitosan exhibit such high viscosities that delivery using a metering pump becomes difficult and not practical. Therefore, this limits the commercial applications of liquid chitosan solutions due to the cost of shipping, which is based on weight. For example, a 3% chitosan solution is essentially ˜97% water and the shipping cost is primarily attributed to water.
An alternative to liquid formulations has been to prepare dry solid chitosan salts that are water soluble. This involves reacting chitosan, dispersed in an organic medium, with an organic acid. The reaction product is then dried, resulting in a dry solid protonated chitosonium salt derivative that is water soluble. Although the production of a dry solid water soluble chitosonium salt mitigates the cost limitations of shipping a 1-3% chitosan solution, there is the production cost to produce the chitosonium salt that still limits the number of potential commercial applications.